Reinforced, three zone microporous membrane

ABSTRACT

A three zone, reinforced, continuous, geometrically symmetrical microporous membrane including a porous support material encapsulated within a middle zone disposed between an upper zone and a lower zone wherein at least one of the three zones has a pore size at least about twenty (20%) percent greater than the pore size of the other two zones is disclosed. Apparatus and methods for fabricating three zone reinforced, continuous, geometrically symmetrical, microporous membrane are also disclosed.

RELATED APPLICATION

This application is related to commonly owned U.S. Provisional PatentApplication Ser. No. 60/043,181, filed Apr. 11, 1997, of Meyering etal., the disclosure of which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to continuous, reinforced, geometricallysymmetrical, microporous membranes having three distinct pore zones andto processes of making and using same, more particularly to reinforcedmicroporous membranes including a scrim having two sides at leastsubstantially encapsulated within a first dope and at least oneadditional dope coated onto each side of the encapsulated scrim prior tothe first dope being quenched and, most particularly, to a geometricallysymmetric, continuous, reinforced membrane having three distinct porezones including a scrim at least substantially and preferably,completely encapsulated by a relatively large pore size middle zone andtwo outer zones, one on each side of the middle zone, at least one ofthe three zones having a pore size at least about twenty (20%) percentgreater than the other zones.

Microporous phase inversion membranes are well known in the art.Microporous phase inversion membranes are porous solids which containmicroporous interconnecting passages that extend from one surface to theother. These passages provide tortuous tunnels or paths through whichthe liquid which is being filtered must pass. The particles contained inthe liquid passing through a microporous phase inversion membrane becometrapped on or in the membrane structure effecting filtration. A slightpressure, generally in the range of about two (2) to about fifty (50)psid (pounds per square inch differential) is used to force fluidthrough the microporous phase inversion membrane. The particles in theliquid that are larger than the pores are either prevented from enteringthe membrane or are trapped within the membrane pores and some particlesthat are smaller than the pores are also trapped or absorbed into themembrane pore structure within the pore tortuous path. The liquid andsome particles smaller than the pores of the membrane pass through.Thus, a microporous phase inversion membrane prevents particles of acertain size or larger from passing through it, while at the same timepermitting liquid and some particles smaller than that certain size topass through. Microporous phase inversion membranes have the ability toretain particles in the size range of from about 0.01 or smaller toabout 10.0 microns or larger.

Many important micron and submicron size particles can be separatedusing microporous membranes. For example, red blood cells are abouteight (8) microns in diameter, platelets are about two (2) microns indiameter and bacteria and yeast are about 0.5 microns or smaller indiameter. It is possible to remove bacteria from water by passing thewater through a microporous membrane having a pore size smaller than thebacteria. Similarly, a microporous membrane can remove invisiblesuspended particles from water used in the manufacture of integratedcircuits in the electronics industry. Microporous membranes arecharacterized by bubble point tests, which involve measuring thepressure to force either the first air bubble out of a fully wettedphase inversion membrane (the initial Bubble Point, or “IBP”), and thehigher pressure which forces air out of the majority of pores all overthe phase inversion membrane (foam-all-over-point or “FAOP”). Theprocedures for conducting initial bubble point and FAOP tests arediscussed in U.S. Pat. No. 4,645,602 issued Feb. 24, 1987, thedisclosure of which is herein incorporated by reference. The procedurefor the initial bubble point test and the more common Mean Flow Poretests are explained in detail, for example, in ASTM F316-70 and ANS/ASTMF316-70 (Reapproved 1976) which are incorporated herein by reference.The bubble point values for microporous phase inversion membranes aregenerally in the range of about two (2) to about one hundred (100) psig,depending on the pore size and the wetting fluid.

U.S. Pat. No. 3,876,738, the disclosure of which is herein incorporatedby reference, describes a process for preparing microporous membranes byquenching a solution of a film-forming polymer in a non-solvent systemfor the polymer. U.S. Pat. No. 4,340,479, the disclosure of which isherein incorporated by reference, generally describes the preparation ofskinless microporous polyamide membranes by casting a polyamide resinsolution onto a substrate and quenching the resulting thin film ofpolyamide.

Since the mechanical strength of some microporous membranes is poor, itis known to reinforce such membranes with a porous support material toimprove mechanical properties and facilitate handling and processing.Accordingly, the aforementioned U.S. Pat. No. 4,340,479 describes aprocedure wherein a polymer solution is directly cast onto a poroussupport material so that the polymer solution penetrates the supportmaterial during casting and becomes firmly adhered thereto duringformation of the reinforced inner layer of a composite microporousmembrane. The support material preferably possesses an open structure sothat pressure drop across the composite membrane is minimized. U.S. Pat.No. 4,340,479 further discloses combining two or more microporousmembranes, one of which may be reinforced, to form a dual or triplelayered structure which is dried under conditions of restraint toproduce a single sheet having particle removal characteristics superiorto those of individual layers.

U.S. Pat. No. 4,707,265, the disclosure of which is herein incorporatedby reference, discloses a reinforced laminated filtration membranecomprising a porous reinforcing web impregnated with a polymericmicroporous inner membrane and at least one polymeric microporous outerqualifying membrane laminated to each side of the impregnated web. Thepore size of the inner membrane is greater than the pore size of theouter membranes. In this manner, the imperfections, e.g., fiber bundles,broken fibers, void areas, and the like, which are invariably present inthe reinforcing web are confined to a coarse, more open inner membraneand the tighter outer qualifying layers are strengthened and supportedby the web. The qualifying layers are not affected by imperfectionspresent within the reinforcing web. Further, the use of a coarse, largepore size inner membrane layer insures that there is no substantialpressure drop of fluid across the reinforcing web.

The membranes disclosed in U.S. Pat. No. 4,707,265 are complicated andcostly to produce since three separate operations are required toproduce the composite membrane: first, the impregnated reinforcedmembrane support layer is produced, second, the non-reinforcedqualifying layers are produced and, third, the impregnated reinforcedmembrane support layer and the non-reinforced qualifying layers arelaminated to form the multilayer composite microporous membrane.

Due to processing and handling restraints, there is a limit to how thinthe impregnated reinforced membrane support layer and the non-reinforcedqualifying layers can be. As a result, the multilayer compositemicroporous membrane of U.S. Pat. No. 4,707,265 is at least about ten(10) mils thick. Furthermore, the overall pore size of the compositemembrane described in U.S. Pat. No. 4,707,265 is generally limited tothe range of approximately 0.45 microns or lower due to the difficultiesof separately producing and handling non-reinforced qualifying layershaving pore sizes of as high as about 0.45 micron. Thus, the utility ofthe laminated composite membrane is limited to sterilizing applicationsand other applications where membranes having about 0.65, 0.8, 1.2, 3.0and greater micron ratings are not needed.

As the thickness of a membrane increases, pressure drop increases, flowrate worsens and the performance characteristics of the membrane areadversely affected. For example, with increasing thickness the totalnumber of pleats in a pleated cartridge element decreases, therebyreducing the effective surface area available for filtration.Furthermore, a mechanical strain exists at the crest of each pleat andincreases with increasing thickness. As a result, thick membranes aremore likely to crack during the pleating, edge-seaming, etc. operationsthat are attendant to the production of pleated filter cartridgeelements or during oxidative hydrolytic exposure or multiple steamcycling. Therefore, mechanical strains, which can never be fullyrelieved after cartridge fabrication, may decrease the useful life ofthe product and may lead to early failure in integrity.

U.S. Pat. No. 4,770,777 overcomes some of the shortcomings of theprocess disclosed in U.S. Pat. No. 4,707,265 by completely saturatingthe reinforcing web with a large pore size (coarser) membrane castingsolution, applying a small pore size membrane casting solution on oneside of the coated web and then quenching the large and small pore sizecasting solutions from only one side to provide a continuous,geometrically asymmetric membrane possessing a pore size gradient. Thus,the lamination step of U.S. Pat. No. 4,707,265 is eliminated, along withthe necessity of handling the fragile non-reinforced qualifying layers.Further, following the teachings of this patent, it is not possible toapply another casting solution on the other side of the large pore sizereinforced web containing layer. Thus, the only additional layers can becast on top of the second layer cast on the first layer that includesthe woven material. Additionally, the membrane taught in U.S. Pat. No.4,770,777 is a skinned membrane. Accordingly, such membrane suffers fromdrawbacks associated with skinned microporous membranes, in particular,high pressure drop, poor structural integrity, susceptibility to skinbreach, propensity to becoming fouled by debris, etc.

U.S. Pat. No. 5,433,859 attempts to address some of the deficiencies, inparticular, high pressure drop, of the skinned membrane disclosed inU.S. Pat. No. 4,770,777 by proposing, preferably, an incompleteimpregnation of the reinforcing web with coarse membrane castingsolution so that a portion of the reinforcing web having a thickness ofabout 50 microns is not embedded within the microporous membrane. Thelow flow resistance of that portion of the reinforcing web which is notembedded within the microporous membrane ensures that filtered fluidpassing through the supported microporous membrane will not have asignificant adverse impact on the pressure drop across the filtrationelement.

While the membrane disclosed in U.S. Pat. No. 5,433,859 exhibits lowerpressure drop across the membrane compared to the skinned membranedisclosed in U.S. Pat. No. 4,770,777, the membrane does have significantstructural drawbacks. First, the membrane suffers from tremendousgeometric asymmetry around the central axis of the reinforcing web,i.e., the thickness of the membrane varies on each side of thereinforcing web. As a result, when the membrane is pleated, themechanical strain on the thick side of the membrane is greater than onthe thin side of the membrane. This differential in mechanical strainincreases the possibility of stress crack formation and failure of theintegrity of the membrane. Second, the membrane poses a possible risk ofseparation along the membrane-reinforcing web interface, especiallyduring backwashing operations. Third, the membrane exhibits “sidedness”having a different pore size on one side versus the other side and anexposed scrim reinforcement area. This will limit its utility in certainapplications such as analytical, or some diagnostic filtrationtechniques. Finally, as with the U.S. Pat. No. 4,720,777, the membraneof the U.S. Pat. No. 5,433,859 cannot have another section on theopposite side of the membrane-reinforced web for the same reason as theU.S. Pat. No. 4,770,777.

Thus, there is a need for a relatively thin geometrically symmetrical,continuous, monolithic, reinforced, polymeric microfiltration membranehaving at least three independent and distinct pore size performancezones (one reinforced performance zone, presently preferably, central tothe membrane structure, and two outer non-reinforced performance zonesincluding at least one outer qualifying performance zone on one side ofthe central reinforced zone and a second outer non-qualifying prefilterperformance zone on the other side of the central performance zone or,two outer qualifying performance zones, one on each side of the centralzone) progressing through the thickness of the membrane, each zone beingcontinuously joined throughout the membrane structure. The three zonesshould be continuously joined by the molecular entanglement which occursin the liquid state of the dope after the dope of each outer zone iscoated onto the dope of the central zone prior to quenching and not by alamination bond after quenching. Such a three zone membrane structureshould be produced by a highly robust, single unit operation, withon-line pore size and layer thickness attribute control. Such a threezone membrane should meet the industries long recognized need forsuperior performance and greater flexibility of triple layer compositestructures. Such a three zone membrane should be relativelyinexpensively and easily manufactured. Such a three zone membrane shouldsimplify the production of traditional laminated single layer structuremembrane and increase the range of pore sizes and manageable handlingthickness that are provided by the non-reinforced zones. Such a threezone membrane should avoid the highly geometrically asymmetric structureof the two zone prior art membrane. Such a three zone membrane shouldhave a geometrically symmetric structure having improved utility,flexibility, and processability into finished industrial forms (pleatedcartridges, etc.) while assuring structural integrity. Such a three zonemembrane should possess a surprisingly thin cross section, having threeindependent performance zones in a geometrically symmetrical,continuous, monolithic, reinforced, polymeric, microfiltration membrane.Such a three zone membrane should have a robust mechanical strength,suitable for pleating and industrial handling and capable of beingproduced on-line and in real time in a surprisingly wide range of poresize attributes, when the apparatus of the present application iscoupled with the commonly assigned copending application Ser. No.09/022,295, filed Feb. 11, 1998, now U.S. Pat. No. 6,056,529, entitled“Methods and Systems For Producing A Plurality of Different MicroporousPhase Inversion Membrane Each Having Any One Of A Plurality Of DifferentPore Sizes from a Single Master Dope Batch”, the disclosure of which isherein incorporated by reference. Such a three zone membrane should havea minimum functional thickness providing maximum throughput at minimalpressure drops, high integrity and be economically produced in a singlemanufacturing operation.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a three zone,reinforced, continuous, non-laminated, geometrically symmetricalmicroporous membrane possessing structural integrity.

Another object of the present invention is to provide a three zone,reinforced, continuous, non-laminated symmetrical microporous membraneexhibiting low pressure drop and high flow rate across the membrane.

A further object of the present invention is to provide a three zone,reinforced, continuous, non-laminated, geometrically symmetricalmicroporous membrane which is particularly suitable for the filtrationof biological or parenteral fluids.

Yet a further object of the present invention is to provide a threezone, reinforced, continuous, non-laminated, geometrically symmetricalmicroporous membrane which is particularly suitable for the filtrationof high purity water for the electronics industry.

Yet another object of the present invention is to provide a method forfabricating such a three zone, continuous, reinforced, non-laminated,geometrically symmetrical microporous membrane.

In accordance with these and further objects, one aspect of the presentinvention includes a three zone microporous membrane comprising: aporous support material substantially impregnated by a first dope toform a middle zone having two sides; and a second zone and a third zoneformed from at least one additional dope, each zone having inner andouter surfaces, each of the second and third zones being operatively,continuously, connected to opposite sides of the middle zone, wherein atleast one of the three zones has a pore size at least about twenty (20%)percent greater than the pore size of at least one of the other zones.

Another aspect of the present invention includes a three zonereinforced, continuous, geometrically symmetrical microporous membranecomprising: a porous support material; and a continuous microporousmembrane having a middle zone disposed between an upper zone and a lowerzone, each having an outer surface, wherein the support material issubstantially embedded within the middle zone and at least one of thezones has a pore size at least about twenty (20%) percent greater thanthe pore size of at least one of the other zones.

Another aspect of the present invention includes a three zonemicroporous membrane prepared by a process comprising: the steps of:providing a continuous support material; at least substantially pressureimpregnating the support material with a first dope utilizing a firstdie means; passing the dope impregnated continuous support materialbetween substantially opposed second and third die means; andsubstantially, simultaneously coating both sides of the dope impregnatedcontinuous support material with at least one additional dope on eachside of the substantially impregnated support material.

Other objects and advantages of the invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of the membrane according to the presentinvention;

FIG. 2 is a schematic representation of the method and apparatus of thepresent invention;

FIG. 3 is an enlarged perspective view of a web positioned between theopposed dies of FIG. 2, with a portion of one die partially broken away.

FIGS. 4a-h are scanning electron photo micrographs of a supported threezone microporous membrane of the present invention illustrating theinterface of the three porous zones at 100×, 300×, 500×, 1,000×, and2,500×; and

FIGS. 5a-h are scanning electron photo micrographs of a supported threezone microporous membrane of the present invention illustrating theinterface of the three porous zones at 100×, 300×, 500×, 1,000×, and2,500×.

DETAILED DESCRIPTION OF THE INVENTION

As illustrated in FIG. 1, one representative, presently preferred, threezone, reinforced, continuous, non-laminated, geometrically symmetrical,microporous membrane 10 comprises a porous support material or scrim 12at least substantially encapsulated in a middle or first zone 16, themiddle zone being disposed between an upper or second 18 zone and alower or third zone 20, wherein the support material 12 is at leastsubstantially embedded within the middle zone 16, the middle zone havinga pore size at least about twenty (20%) percent greater than the poresize of at least one of the upper zone 18 and the lower zone 20. Whilethis structure for the membrane is presently preferred, it should beunderstood that the pore size of the middle zone does not always have tobe the largest pore size and that any one of the three zones could rangefrom the largest to the smallest possible produceable pore size.

Use of the term “microporous membrane” herein is intended to encompassmicroporous membranes having the ability to retain particles in the sizerange of from about 0.01 or smaller to about 10.0 microns and higher.

The term “continuous” as applied to the microporous membrane of thisinvention shall be understood to refer to a microporous membrane whereina continuum exists between the three zones constituting the membrane andthat no break exists between the polymer structure which comprises themiddle zone and that which comprises the upper zone and the lower zoneof the membrane. The microporous membrane structure is continuousstructure even in the presence of the reinforcing scrim, in that thefiber strains of scrim constitute a network between which themicroporous membrane structure is continuous and penetrating. Thereforthe scrim and the microporous membrane form continuous interpenetratingnetworks of their respective polymeric structures.

The term “monolithic” as applied to the microporous membrane of thepresent invention is intended to mean a single unit.

The phrase “geometric symmetry” utilized herein shall be understood torefer to a structure wherein the upper and lower zones of themicroporous membrane possess substantially the same thickness. By“substantially the same thickness,” it is meant that the thickness ofthe upper zone can differ from the thickness of the lower zone, and viceversa, by not more than about twenty-five (25%) percent. It is importantto contrast the way the term “symmetry” is employed herein to the waythe term “symmetry” is employed in U.S. Pat. No. 4,707,265 wheresymmetry is used to refer to pore size symmetry; thus, in U.S. Pat.No.4,707,265 the term applies when the outer qualifying layers possesssubstantially the same pore size. For certain embodiments of thisinvention, pore size symmetry is a highly preferred, but not essential,characteristic of the present inventive microporous membrane.

The term “pore size” as used in this present application shall beunderstood to mean “Mean Flow Pore” as determined by the appropriateASTM-F316-70 and/or ASTM-F316-70 (Reapproved 1976) tests.

Preferably, the microporous membrane of the present invention ishydrophilic. By the use of the term “hydrophilic” in describing themembrane, it is meant a membrane which adsorbs or absorbs water.Generally, such hydrophilicity is enhanced in the presence of asufficient amount of hydroxyl (OH—), carboxyl (—COOH), amino (—NH₂)and/or similar functional groups on the surface of the membrane.Additionally, hydrophilicity is enhanced by micro textural phenomena asdescribed in Knight, Gryte & Hazlett. Such groups assist in theadsorption and/or absorption of water onto the membrane. Suchhydrophilicity is particularly useful in the filtration of aqueousfluids.

Preferred microporous membranes of the present invention are producedfrom nylon. The term “nylon” is intended to embrace film formingpolyamide resins including copolymers and terpolymers which include therecurring amido grouping and blends of different polyamide resins.Preferably, the nylon is a hydrolytically stable nylon possessing atleast about 0.9 moles of amino end groups per mole of nylon as describedin U.S. Pat. No. 5,458,782, the contents of which are incorporated byreference herein.

While in general the various nylon or polyamide resins are allcopolymers of a diamine and a dicarboxylic acid, or homopolymers of alactam and an amino acid, they vary widely in crystallinity or solidstructure, melting point, and other physical properties. Preferrednylons for use in this invention are copolymers of hexamethylene diamineand adipic acid (nylon 66), copolymers of hexmethylene diamine andsebacic acid (nylon 610), homopolymers of polycaprolactam (nylon 6) andcopolymers of tetramethylenediamine and adipic acid (nylon 46). Thesepreferred polyamide resins have a ratio of methylene (CH₂) to amide(NHCO) groups within the range of about 4:1 to about 8:1. The nylonpolymers are available in a wide variety of grades, which varyappreciably with respect to molecular weight, within the range fromabout 15,000 to about 42,000 (number average molecular weight) and inother characteristics.

The highly preferred species of the units composing the polymer chain ispolyhexamethylene adipamide, i.e. nylon 66, having molecular weightsabove about 30,000. Polymers free of additives are generally preferred,but the addition of antioxidants, surface active agents, chargemodifying agents or similar additives may have benefit under someconditions.

The three zone, reinforced, continuous, monolithic, geometricallysymmetrical microporous membrane of the present invention has, as animportant component thereof, the porous support material 12 at leastsubstantially embedded within the middle zone 16 of the membrane 10 forproviding structural strength or reinforcement to the finished threezone membrane. The porous support material 12 may be prepared from anysuitable material in any suitable manner. The support material 12provides the membrane with sufficient strength to withstand the flowpressures encountered during use without deforming to the extent thatthe microporous membrane 10 is damaged. The support material 12 whichcan be employed herein woven materials in a grid or mesh-likeconfiguration as well as nonwoven materials formed by extrusion,lamination, and the like. The support material 12 preferably comprisespolyester, polypropylene, polyethylene, polyamide and polyvinylidenefluoride, although other web producing polymers may be equally suitable.The support material 12 used in conjunction with the present inventionis, presently preferably, formed from fibers of sufficient strength anduniformity, and uniformly dispersed in cross web and machine directionand generally thin for providing a high degree of structural integrityand low pressure drop. For a general discussion of support materialattributes, refer to U.S. Pat. No.4,645,602.

In one presently preferred embodiment, the middle zone 16 of themicroporous membrane 10 should have an average pore size which is atleast about twenty percent (20%) greater, preferably at least aboutfifty percent (50%) greater, more preferably at least about 100%greater, and most preferably at least about 200% greater, than theaverage pore size of at least one of the upper zone 18 and lower zone 20of the membrane and preferably both the upper and lower zones. The poresformed in the middle zone 16 have an average size of about ten (10)microns or less and the average pore size will preferably range fromabout 0.5 microns to about two (2) microns, more preferably from about0.1 to about one (1.0) microns. The middle zone 16 has a pore sizedistribution which is preferably quite narrow in range, although this isnot essential for satisfactory performance.

The middle zone 16 should be as thin as possible so long as it providessufficient structural strength and embeds the support material 12 suchthat, presently preferably, no fibers of the support material protrudefrom the middle zone 16 into either the upper 18 or the lower 20 zone.However, in one preferred embodiment, some strands/fibers of the supportmaterial 12 are contiguous with or slightly protrude into at least theone of the other two zone 18, 20 formed from a tight dope or coatingsolution or into both zones 18, 20 when both zones are formed from atight dope.

It is believed that having a relatively thin middle zone in which atleast some of the scrim is not completely encapsulated within the middlezone may be advantageous in that the thickness of the middle zone willbe kept to a minimum, thus, resulting in a thinner overall finishedmembrane. The thickness of the middle zone will typically range fromabout fifty (50) microns to about one hundred fifty (150) microns andpreferably from about seventy-five (75) microns to about one hundred(100) microns or whatever dope volume is necessary to substantiallyimpregnate the scrim being impregnated at any specific time.

In one presently preferred embodiment, the upper 18 and the lower 20zones of the microporous membrane 10 possess pores which have a sizeproviding the desired filtration efficiency or particle removal.Generally, the average size of the pores of the upper zone and the lowerzone will be about one (1) micron or less, and can typically range fromabout 0.01 microns to about one (1) microns. More preferably, theaverage size of the pores of each zone 18, 20 will range from about 0.2microns to about 0.5 microns. The pore size distribution of the upper 18and lower 20 zones of the microporous membrane 10 is preferably narrow.In a particularly preferred embodiment, the average pore size of theupper zone 18 is substantially the same as the average pore size of thelower zone 20. By “substantially the same”, it is meant that the averagepore size of the upper zone does not differ from that of the lower zone,and vice versa, by more than about twenty-five (25%) percent.

One important feature of one preferred embodiment of the three zone,reinforced microporous membrane 10 of the present invention is that theupper 18 and the lower 20 zones have substantially the same thickness soas to provide geometric symmetry around the central axis of the membrane10. These zones 18, 20 should be as thin as possible in order tominimize the pressure drop across the microporous membrane 10 whilebeing sufficiently thick to yield desired particulate removal. Theindividual thickness of each of the upper and lower zones will generallyrange from about twenty-five (25) microns to about one hundred (100)microns, preferably from about thirty-five (35) microns to about sixty(60) microns. The overall thickness of the reinforced, continuous,monolithic, geometrically symmetrical, microporous filtration membraneof the present invention will generally not exceed about ten (10) mils.

The geometric symmetry of the present inventive microporous membrane 10minimizes mechanical strains, reduces the likelihood of delamination ofthe membrane and generally improves the structural integrity of themembrane. This is particularly important to fan-fold pleated cartridgearrangements, where both sides of the microporous membrane are expectedto bend equally well around the neutral (unyielding) axis of thereinforcing scrim. Such bending should result in an equal distributionof tension and compression forces in the pleat crests and troughs, suchthat neither side is burdened with an excessive tension or compressionload, which would increase the possibility of damage and/or breechfailure of the membrane at the pleat area. Furthermore, the unique thincross-section of the present invention on both sides provides anadvantage, in that the tension and compression forces are minimized asthe absolute radius from the center of the reinforcement to the outsidesurface of the membrane is minimized. However, it should be understoodthat the thickness of one of the upper 18 or the lower 20 zone could beconsiderably thicker than the other and still be within the teachings ofthe present application.

The reinforced microporous membrane 10 may be rolled and stored for useunder ambient conditions. It will be understood that the three zone,reinforced, microporous membrane of the present invention may be formedinto any of the usual commercial forms, such as, for example, discs orpleated cartridges.

For sterile filtration involving biological liquids, the three zone,reinforced, microporous membrane 10 is sanitized or sterilized byautoclaving or hot water flushing. The three zone, reinforced,microporous membrane of the present invention has proven resistant tothis type treatment, particularly when a hydrolytically stable nylon isused as described hereinabove, and retains its structural integrity inuse under such conditions.

The three zone, reinforced, microporous membrane of the presentinvention is easy to handle and readily formed into convolutedstructures, e.g. pleated configurations. By reason of its improved flowcharacteristics it may be employed directly in existing installations,without pumping modifications. Specifically, due to the improved flowrate, the existing pumps would actually operate at lower loads and thuswould most likely have longer useful lives.

The three zone, reinforced, filtration membrane 10 of the presentinvention is characterized by unexpectedly high flow rates for a givendifferential pressure and also characterized by durability, strength,uniformity, lack of pinholes and bubble defects. In many applications,the preferred membranes may be used with either side of the membranefacing upstream.

As illustrated in FIG. 2, one presently preferred method for preparing athree zone, reinforced, continuous, geometrically symmetricalmicroporous filtration membrane according to the present inventionincludes: providing a porous support material 12 having first 22 andsecond 24 sides, presently preferably, pressure impregnating the supportmaterial 12 with a first solution or dope 26, coating a second solutionor dope 28 over the first side 30 of the pressure impregnated supportmaterial 32, coating a third solution or dope 36 over the second side 31of the pressure impregnated support material 32 such that a continuousmicroporous membrane having a middle zone 16 disposed between an upperzone 18 and a lower zone 20 (See FIG. 1) formed from the first 26,second 28 and third 36 dopes, the support material 12 being, presentlypreferably, completely embedded within the middle zone 16 and the middlezone having a pore size at least about twenty percent (20%) greater thanthe pore size of at least one of the upper zone 18 and the lower zone20.

The dopes 26, 28, 36, and quench bath 38 utilized in the fabrication ofthe reinforced microporous membrane 10 herein are conventional innature. The novel arrangement of slot dies 40, 42, 44 to, presentlypreferably, first pressure impregnate the support material 12 with afirst dope and then to coat both sides thereof with other dopes has beenfound particularly effective to produce the membrane 10. A schematicrepresentation of one presently preferred apparatus for fabricating themembrane 10 of the present invention is shown in FIG. 2 and includes afirst die 40 for pressure impregnating the support material or scrim 12and substantially opposed second and third dies 42, 44 for substantiallysimultaneously coating both sides 30, 31 of the initially impregnatedscrim 12 or other apparatus capable of coating the membrane as describedabove.

The three zone microporous membrane 10 of the present invention isgenerally produced by first pressure impregnating the scrim with a firstdope and then coating any one of a plurality of possible dopescontaining a film-forming polymer in a solvent system onto each side ofthe dope impregnated scrim and immediately quenching the dopes 26, 28,36 in a bath 38 comprised of a conventional nonsolvent system for thepolymer. It is presently believed that an important parameterresponsible for development of micropores in the membrane (e.g. poresize) is the solvent system employed with the polymer and the nonsolventsystem used in quenching the polymer film as well as the phenomenadiscussed in the previously mentioned patent application. The selectionof the solvent for the polymer is determined by the nature of thepolymer material used and can be empirically determined on the basis ofsolubility parameters, as is well known and conventional in the art.

The dopes for forming the preferred nylon microporous membrane of thepresent invention, presently preferably, contain nylon polymers in asolvent system for the polymer. The solvent system comprises a mixtureof at least one solvent and one nonsolvent for the polymer. The solventswhich can be used with alcohol soluble nylons include lower alkanols,e.g. methanol, ethanol and butanol, and mixtures thereof. It is knownthat nonalcohol soluble nylons will dissolve in solvents of acids, forexample, formic acid, citric acid, acetic acid, maleic acid, and similaracids.

The nylon dopes after formation are diluted with a nonsolvent for thenylon which is miscible with the nylon solution. Dilution withnonsolvent may be effected up to the point of incipient precipitation ofthe nylon. The nonsolvents are selected on the basis of the nylonsolvent utilized. For example, when water miscible nylon solvents areemployed, water can be the nonsolvent. Generally, the nonsolvent can bewater, methyl formate, aqueous lower alcohols, such as methanol andethanol, polyols such as glycerol, glycols, polyglycols, and ethers andesters thereof and mixtures of any of the foregoing.

The support material 12 having first 22 and second 24 sides may beimpregnated with the first dope 26 by any of a variety of techniques,e.g., roll coating, spray coating, slot die coating, and the like, withslot die pressure impregnating being presently preferred, tosubstantially completely impregnate the support material 12 with thefirst dope 26. As used in this disclosure, “complete impregnation of thesupport material” means that all fibers of the support material arecompletely surrounded by liquid dope and that no portion of the supportmaterial is not covered by liquid dope and that no portion of thesupport material protrudes from the center zone into either the secondor third zones in the finished three zone membrane.

The support material 12 is preferably maintained under tension, in amanner known in the art, while the first dope 26, under pressure,penetrates and saturates the support material 12. The impregnatedsupport 32 can be calendered, if desired, by rollers to force the firstcoating solution into the support as described in U.S. Pat. No.4,707,265, the contents of which are incorporated by reference herein.Thereafter, the second dope 28 is coated over the first side 30 of theimpregnated support material 32 and the third dope 36 is coated over thesecond side 31 of the impregnated support material employing thesubstantially opposed slot dies or any other suitable technique whichprovides for the essentially simultaneous coating of a second dope onone side of the dope impregnated scrim and a third dope on the secondside of the dope impregnated scrim. Application of the second 28 andthird dopes 36 is, presently preferably, simultaneous or substantiallysimultaneous using substantially opposed slot dies 42, 44 such that theimpregnated solution 32 is supported by the mutual hydrodynamic forcesof the substantially opposed slot dies 42, 44. Slot dies 42, 44 to whichthe dopes 28, 36 are fed under pressure have been found to provideparticularly good results in applying the second 28 and third 36 dopesto the sides of the dope impregnated support member 12. Preferably, theslot dies 42, 44 are disposed essentially directly opposite one another(See FIG. 2) with the dope impregnated support 32 passing therebetween.The second 28 and third 36 dopes are coated over each side 30, 31 in,presently preferably, substantially equal amounts but are not requiredto be coated with equal amounts of dope.

In accordance with one preferred embodiment, the second 28 and third 36dopes produce substantially identical pore sizes but produce a differentpore size from the first dope 26. In accordance with another preferredembodiment, the second 28 and third 36 dopes produce a different poresize as well as each producing a different pore size from the first dope28. It is possible to have any pore size from the largest to thesmallest in any of the three zones and in any order.

Thereafter, the first 26, second 28 and third 36 dopes aresimultaneously quenched with the outer surfaces of the second and thirddopes having direct contact with the quenching fluid in the same quenchbath 38. Since the first 26 dope is, presently preferably, more coarse,it will coagulate more slowly, provide for the formation of a threezone, continuous, monolithic, symmetrical, geometrical, microporousmembrane 10 having a relatively open-pore middle zone 16 (See FIG. 1)disposed between tighter pore sized upper zone 18 and lower zone 20 or arelatively open-pore sized upper or lower zone and a tighter pore sizedupper or lower zone. After the microporous membrane is formed, themembrane is washed and dried to provide the final product, shown in FIG.1.

It has been determined that the shape of the nose of the first die 40used to pressure impregnate the scrim 12 is important to accomplishingsubstantially complete and, presently preferably, complete impregnationor saturation of the scrim 12. Specifically, in order to obtain acomplete or a substantially complete impregnation of the scrim 12, therelative position of the scrim on the nose of the die 40 should beapproximately as depicted in FIG. 2, except that the upper die surfacecontacting the scrim should be arched instead of being straight, asshown. Specifically, it is believed that the upper portion of the die 40should be, presently preferably, arched with the angle that the scrim 12forms with the die should be between about five (5°) degrees and aboutsixty five (65°) degrees. Since it is important for one aspect of theinvention that the first dope 26 substantially completely impregnate thescrim 12, this relative position of the scrim to the upper surface ofthe die 40 has been determined as important to ensure that not only isthe scrim completely or substantially completely impregnated andsaturated with the first dope but also that, presently preferably, asufficient amount of liquid dope extends beyond the fibers of the scrim12 so that all fibers are covered by, presently preferred, at leastabout one (1) mil of liquid dope prior to the scrim, impregnated withthe first liquid dope, having the second and third coating dopes coatedsubstantially simultaneously thereon.

Further, since it is important to prevent or at least minimize vaporsfrom the quench bath from contacting the dopes after the scrim has beenimpregnated and coated on both sides with the dope, means, such as, forexample, a controlled vapor zone, are provided for preventing or atleast minimizing the quench bath vapors from interacting with the coatedscrim before quench. This controlled vapor zone is needed to preventdope from solidifying on the bottom of the dies and to prevent quenchingof the dope from contact with the vapors before the dope reaches thequench bath, as is known in the art.

However, it is also important in another aspect of the invention topressure impregnate a predetermined amount of the first dope into thescrim such that at least one portion of the scrim is not completelycovered by the liquid dope. In such cases, at least one fiber or portionof a fiber of the scrim be at least contiguous with or slightly protrudeinto the second dope zone and/or the third dope zone. When producing athree zone membrane having the same pore size zones on both sides of thecenter zone, both sides of the dope impregnated scrim have at leastportions that are at least contiguous with or protrude above the liquiddope surface after the scrim has been impregnated thereby. Suchincomplete encapsulation of the scrim by the first dope results infinished three zone membrane having portions/fibers of the scrim thatprotrude or are contacted by both the second and third zones or only oneof the zones, the zone having the tighter pore size.

When producing a three zone membrane having three different pore sizedopes, it is presently preferred that the side of the pressureimpregnated scrim that is coated by the tight dope have at least oneportion of the scrim extending above the level of the dope impregnatingthe scrim, breaking the surface tension of the liquid dope substantiallyimpregnating the scrim after the scrim has been pressure impregnated andbefore being coated by the tight dope.

The described method can be conducted in a continuous or batch-wisemanner in a number of representative apparatus. In general, the supportmaterial 12, e.g., in the form of a nonwoven fibrous scrim, is unwoundunder tension from a roll and pressure impregnated with the first dope26 as described above. The pressure impregnated support material 12 isthen coated on each side 30, 31 with second and third dopes aspreviously described. The unquenched dope/scrim combination is thensubstantially immediately immersed while still under tension in a quenchbath to form the three zone, continuous, microporous membrane of thepresent invention from the first, second and/or third dopes. Themicroporous membrane is then dried and wound under tension on a roll forstorage, as is known in the art.

It is believed that impregnation of the scrim is a function of theviscosity of the dope, the back tension on the scrim, the gap in theinitial die which effects the dope pressure and the velocity of thescrim relative to the dope. Each of these parameters is unique to thespecific scrim being impregnated by the dope and can be determined bythose skilled in the art.

As an example, as would be appreciated by those skilled in the art, ifthe viscosity of the first dope is too low, the first dope will lackcohesiveness and the ability to be readily coated by the second and/orthird dopes. If too viscous, the first dope will not completely,properly, impregnate the scrim which will cause an excess of the firstdope to remain on the die side of the scrim and not properly penetrateto the far said of the scrim.

As illustrated in FIG. 2, one representative apparatus 50 useful in theproduction of the membrane of the present invention includes aconventional assembly 52, for providing the continuous scrim or othersupport structure 12 for receiving the polymer dopes 26, 28, 36. Theconventional assembly typically includes an unwind station comprising alet-off apparatus which includes a spindle for mounting one or morerolls of support material and related release and brake elementsconventionally employed for paying out a continuous sheet of the supportmaterial, as is known in the art. The assembly 52 also includes aplurality of conventional unwind rollers which begin to orient movementof the scrim through a ladder unit which conventionally includes aseries of rollers which further align and begin to tension the scrim 12and prepare the scrim for the impregnation operation, as is known in theart.

After the scrim 12 leaves the conventional ladder unit, the scrim entersa conventional drive section. The drive section includes a plurality ofindividual rollers, at least one of which is driven to pull the scrim 12from the conventional unwind station. Additionally rollers are providedand arranged to regulate the tension in the scrim 12 and the position ofthe scrim 12, as is known to those skilled in the art.

The scrim 12 is fed by the conventional drive section, downwardlybetween, presently preferably, a series of dies, including the first die40 for, presently preferably, completely pressure impregnating the scrimwith a first dope 26 and second 42 and third 44 dies for coating asecond 28 and a third 36 dope on to the outer surfaces of the dopeimpregnated scrim. In the preferred embodiment of one apparatus usefulto produce the present invention, the first die 40 is a single slot die,operatively connected to a suitable reservoir 60 containing the firstdope 26. The first dope may vary depending on the type of film-formingpolymer used, but is generally a liquid dope formulated and treated toproduce a specific pore size when quenched. A conventional controlledpumping mechanism (not shown) operates to selectively deliver the firstdope 26 from the reservoir 60 to the first die 40. The first die 40 hasan opening configured to provide an even amount of the first dope 26 soas to pressure impregnate the scrim 12 as the scrim 12 passes by theopening of the first die 40. When different sizes of scrim 12 are used,the die 40 may be changed for appropriate scrim impregnation. It isimportant that the dope 26 transferred to the scrim 12 substantiallycompletely saturate or impregnate the scrim, as was discussed above.

After the scrim 12 is at least substantially impregnated or saturatedwith the first dope, the scrim travels between the second 40 and third44 dies. In one embodiment of the apparatus, the scrim 12 is disposedvertically and travels in the downward direction. In one presentlypreferred embodiment of the apparatus, the scrim 12 may initially travelat an angle less than vertical, as shown in FIG. 2. Second 40 and third44 dies are essentially disposed on opposite sides of the scrim 12 inorder to produce the membrane of the present invention. Second die 42 isdirected to coat the polymer dope 28 desired onto the first surface 22of the substantially saturated scrim 12 and in like manner, third die 44is directed to coat the polymer dope 36 desired onto the second surface24 of the substantially saturated scrim 12. Each die 42, 44, is fed froma reservoirs 62, 64 having the dopes 28, 36. The dopes comprise, forexample, nylon 66 dissolved in formic acid where the desired polymermembranes are nylon and identical. It is to be appreciated that thedopes may be a combination of any of the well-known film-formingpolymers in an appropriate well-known solvent. Conventionally controlledpumping mechanisms (not shown) selectively deliver the dope 28, 36 tothe dies 42, 44.

As best shown in FIG. 3, the dies 42 44 are each disposed on oppositesides of the pressure impregnated scrim 12 and essentially opposed tothe other die. Each die 42, 44 has a chamber 72 for receiving the dopesolution and a narrow slot 74, transversely extending across each sideof the front 75 of each die, for transferring the dope solution onto theimpregnated scrim 12 (die 40) and then to coat the substantiallysaturated scrim on both sides (dies 42, 44). The dope is forced out ofthe slots 74 by the pressure supplied by the conventional reservoirpumps (not shown), in a manner known in the art. The pressure providedto the dope varies with each dope and scrim used. Determination of theappropriate pressure for any of the dopes applied to a particular scrimis known to those skilled in the art. The dies 42, 44 are positionedclose enough to the substantially saturated, impregnated scrim 12 sothat the dope directly contacts the outer surface 22 of the dopesaturated scrim 12 when the dope is forced from the slot 74. As isapparent in FIG. 3, the length of the slot 74 determines the final widthof the dope coated onto the saturated scrim. By masking or otherappropriate means, it is possible to foreclose coating the dope at theedges of scrim 12, leaving a selvage area 76 for trimming, potting orother post-formation operations. It is to be understood that the initialdope is different from the other dope(s) and that it is possible to havethree different dopes, with a first dope impregnating the scrim 12 andthe second and third dopes coated on each side of the first dopeimpregnated scrim, resulting in a graded density three zone membrane.

In similar fashion, although not shown, intermediate areas along theslots 74 can also be masked to accommodate the ultimate filtrationpurposes and apparatus in which the reinforced, continuous membrane ofthe present invention is to be employed. The internal configuration ofthe first die 40 is similar and therefore has not been disclosed ingreater detail. However, it is believed important that the first die 40be positioned so that the initial dope substantially, completelysaturates the scrim 12, as will be appreciated by those skilled in theart.

As shown in FIG. 2, after all three dopes have been applied to the scrim12, the resulting unquenched scrim reinforced structure is directed intothe quenching unit 38. Quenching unit 38 is conventional and includes aconventional reservoir for circulating a quantity of nonsolvent,hereinafter referred to as the quench bath, for the dissolved polymerwhich forces the polymer in each of the three dope zones to solidify.The result of the quench is a continuous, non-laminated, geometricallysymmetric, reinforced, membrane 10 comprising a zone of microporouspolymer 18, 20 on each side of a middle zone 16 of microporous polymerencapsulating a support material 12 (See FIG. 1). After the polymershave solidified in the quench, the membrane 10 passes over aconventional first roller that is immersed in the quench bath. Themembrane 10 is then conventionally drawn through the quench bath andaround a second roll which is driven by conventional drive means (notshown). At this time, formation of the composite membrane 10 iscomplete, but excess liquid from the quench bath 38 remains thereon. Theresulting three zone membrane 10 is geometrically symmetric because thelayers 18, 20 of polymer were substantially equally coated and quenchedbefore contacting any rollers or other devices that might interfere withthe solidification of the dope polymers during quenching.

As shown in FIG. 2, the scrim 12 having three distinct layers of dopeoperatively applied thereto is directly immersed in the quench bath 38.For the purpose of this disclosure, the term directly is intended tomean that the impregnated, coated scrim does not contact or interactwith any rollers or other solid elements of the apparatus 50 between thedies 40, 42, 44 and the quench bath 38. Thus, directly is not intendedto refer to the length of time that the impregnated, coated scrim takesto travel from the coating dies 42, 44 to the quench bath and is notintended to refer to the physical distance between the coating dies 42,44 and the quench bath 38. However, it is preferred that the distanceand the time be as short as possible consistent with the production ofhigh quality membrane. Further, since it is important to prevent or atleast minimize vapors from the quench bath from contacting the dopesafter the scrim has been impregnated and coated on both sides with thedope, means, such as, for example, a controlled vapor zone, are providedfor preventing or at least minimizing the quench bath vapors frominteracting with the coated scrim before quench. This controlled vaporzone is needed to prevent dope from solidifying on the bottom of thedies and to prevent quenching of the dope from contact with the vaporsbefore the dope reaches the quench bath, as is known in the art.

The newly formed membrane 10 is presently preferably, immediately rinsedof excess fluid from the quench in a conventional first stage rinsingunit 70, as is known in the art. The membrane is thereafter, directedover another plurality of rollers and into a counter-current flow washtank 72, including a reservoir containing a quantity of water, aplurality of rollers to increase the contact time of the membrane 10within the tank 72, and suitable spraying and circulation apparatus, asknown in the art to complete the rinse of the membrane 10, as is knownin the art. After the membrane 10 leaves the wash tank 72, it enters aconventional winding section 74 where the membrane 10 is wound onto aspindle or the like for storage and drying, as is known in the art.

As should be apparent from the drawings and the previous description,the dies 42, 44 are disposed in opposed fashion to coat, presentlypreferably, simultaneously, both surfaces of the substantially saturatedscrim which, in turn, is passing vertically therebetween. Thesubstantially saturated scrim coated on both sides by dope emanatingfrom the dies is then caused to pass a predetermined distance, towardthe quenching unit downwardly where the impregnated, coated scrim iscontacted only by air. The distance can be controlled somewhat bymovement of the dies 40, 42, 44 and more readily by lowering or raisingthe level of the quench liquid in the tank. Control over this distancemay effect formation of the microporous membrane by controlling thevapor zone.

Once having traversed the distance to the quench tank, the impregnated,coated scrim is then immersed in the quench fluid contained therein. Thecoated scrim is then caused to pass a predetermined distance, within thequenching unit, before reaching a first roller, as is known in the art.

An important aspect of the method for producing the product of thepresent invention is that the impregnated, coated scrim does notencounter any rollers or other solid or physical elements of theapparatus at this stage, which is prior to solidification of the threezones of dope to an extent that the membrane develops sufficientintegrity to avoid and resist deformation encountered during subsequentsteps of the manufacturing process. Accordingly, the first predetermineddistance and the second predetermined distance function together toprovide means for permitting the polymer membranes to solidify on theimpregnated, coated scrim sufficiently to avoid and resist damagingdeformation during subsequent manufacture of the composite membrane.This ensures that the membrane zones, 18, 20 are substantially uniformin thickness and provide the pore structure and size desired andintended by selection of the dope(s) and quench solutions and otherparameters including temperature, concentrations, rate of theimpregnated, coated scrim through the apparatus and the like.

Generally, the residence time the impregnated, coated scrim travelswithin the quench tank 38 is related to speed of travel of theimpregnated, coated scrim, temperature and concentration of the quenchfluid and the height of the tank Accordingly, at the bottom of the tank38, a roller, as is known in the art, is provided to reverse the traveldirection of the coated scrim, upwardly and out of the tank.

Upon exiting the tank 38 the quenched membrane is subjected to washingfor the purpose of removing the excess quench liquids. The apparatusprovides first state rinsing unit 70 and a counter-current flow washtank 72, as described hereinabove. Thereafter, the membrane structure iswound and/or dried for subsequent usage, as is known in the art.

EXAMPLES

Preparation of the dopes:

Two dopes were prepared using the methods described in U.S. Pat. No.4,707,265, Example 1. The dopes were produced using a 14.5 percent byweight Nylon 66 (Monsanto/Solutia Vydyne® 66B) polymer. Thecharacteristics of the prepared dopes processed as standard dry doublelayer non-reinforced membrane are given in Table I.

TABLE I Dopes for Examples 1, 2, and 3 Dope % Viscosity, IBP FAOPThickness Q M.F.P. Dope I.D. Type Solids (cp) (psig) (psig) (mils)(cc/min) (micron) 97L028 “A” = Smaller 14.5 ˜3000 43.5 49.8 7.0  69.10.426 Pore 97L038 “B” = Larger 14.5 ˜3000 19.7 21.6 9.1 259.2 1.006 Pore

Example 1

A geometrically symmetric and pore size symmetric reinforced three zonemembrane, with an “open” (large pore size) scrim impregnation wasprepared as follows.

A non-woven Polypropylene bicomponent fiber web or scrim suitable forpreparation of the present invention (commercially available fromFreudenberg under the tradename Viledon®, Grade # F02432), having abasis weight of nominally 30 gm/sq.meter was processed by the methodtaught in the present application. The scrim was pre-treated with a mildCorona Discharge to enhance it's wetabiliy prior to being pressureimpregnated. The larger pore size dope, 97L038, was used to pressureimpregnate the web, with an impregnation weight of about seven (7)gm/sq.meter of Nylon solids. The nylon solids were provided from thedissolved nylon in the dope solution, which was, for this example, afourteen and one half (14.5) wt % nylon solution (approximately 50 gramsof liquid dope per square meter), which was sufficient to impregnate andfill the void volume of the scrim, creating the first zone of large poresize dope integral with the supporting scrim. Almost immediatelyfollowing the pressure impregnation of the scrim with the 97L038 dope,both sides of the pressure impregnated scrim were essentiallysimultaneously coated with substantially even layers of the small poresize dope, 97L028. In this example, the total coating weight deliveredto the two sides was about thirty seven (37) gm/sq.meter of Nylon solidsin about a fourteen and one half (14.5) wt % solution (approximately 260grams of liquid dope per square meter), with the total being split intotwo streams of dope feeding onto the two sides, so that both sides weresubstantially evenly coated with the same dope, creating the second andthird zones of small pore size dope. The split in the amount of the97L028 dope was not perfect, in that one side of the impregnated scrimreceived approximately fifteen (1 5) gm/sq.meter of Nylon solids (Zonetwo), where the other side received approximately twenty two (22)gm/sq.meter of Nylon solids (Zone three). The imbalance in the amount ofdope coated on the two sides resulted in a slight imbalance in the smallpore size qualifying zone coating, but the imbalance was not detrimentalto the performance of the finished product. The grand total applicationof both dopes (large and small pore size) was, thus, approximately fortyfour (44) gm/sq.meter Nylon solids. The thus coated three zone structurewas then quickly brought into contact with a Marinacco-style quenchsolution, which simultaneously quenched the three zone structure fromthe outer surfaces of the small pore size dope, 97L028, such that acontinuous microporous membrane structure was formed. The quenchedmembrane was then washed, dried under X & Y direction dimensionalrestraint, and tested, in the usual manner. The test results are shownin Table II.

FIGS. 4a-4 f are Scanning Electron Photo Micrographs of a cross-sectionof the membrane produced in Example 1.

Example 2

A geometrically symmetric and pore size asymmetric three zone membranewas prepared as follows.

A second three zone membrane was prepared in nearly identical manner asin Example 1, with the exception that one of the coating sides of thepressure impregnated scrim (in this case, Zone two) was coated with thesame approximately fifteen (15) gm/sq.meter Nylon solids from the largepore size dope 97L038. The opposite side (Zone three) was coated withthe approximately twenty two (22) gm/sq.meter Nylon solids from smallpore size Dope 97L028. After two-side simultaneous quenching, washingand restrained drying, the resultant finished membrane had achieved acontinuous, substantially geometric symmetry around the neutral axis ofthe reinforcing scrim, but had very different pore size attributes onboth sides of the scrim. (i.e., Pore Size Asymmetric.) The test resultsfor this membrane are also shown in Table II.

FIGS. 5a-5 f are Scanning Electron Photo Micrographs of a cross-sectionof the membrane produced in Example 2.

Example 3

A Control, Reinforced Membrane (single dope, three zones) was prepared.

A control reinforced membrane was produced for comparison with thereinforced membrane produced according to the method of the presentapplication. This three zone, reinforced, membrane was identical to themembrane produced in Example 1, except that the pressure impregnatedfirst zone was also produced using the small pore size dope 97L028.Thus, all three zones were produced using a single dope, split intothree streams to each of the dies. After two-side simultaneousquenching, washing and restrained drying, the resultant finishedmembrane was a continuous, substantially geometrically symmetric, singlepore size structured membrane; which was similar in appearance andfunction to any standard single layer reinforced membrane which iscommon to the Nylon microporous membrane industry today. The testresults for this membrane are also shown in Table II.

TABLE II Membrane Test Attributes from Examples 1, 2, and 3 Dope Type inZone IBP FAOP Thickness Q M.F.P. Example # “3-1-2” Roll I.D. # (psig)(psig) (mils) (cc/min) (micron) 1 “A-B-A” 97L028-05 44.2 54.7 7.1 970.430 2 “A-B-B” 97L028-03 41.3 47.9 7.3 145.1 0.562 3 “A-A-A” 97L028-0141.8 49.7 6.9 81.4 0.489 (Control)

Discussion of Examples 1 through 3

As can be seen from Table II, the Example 1 membrane has a clearlyimproved flow rate over the standard (control) membrane. The raw waterflow rate (Q, expressed as cc/min clean deionized water for a nominallyforty seven (47) mm test disc (13.5 cm² test area) under water pressureof 5 psid) has shown about a twenty (20%) percent improvement, while theintegrity, as measured by Initial Bubble Point, has surprisinglyincreased by about six (6%) percent, for the same overall membranethickness. This improvement potentially provides a double benefit, thesebeing improved clean water flow rate and improved integrity as measuredby IBP. The increase in Initial Bubble Point is corroborated by both theincrease in membrane Foam-All-Over-Point, and the decrease in the ASTMMean Flow Pore size rating.

The Example 1 membrane is representative of the advantage of the presentinvention, where there are two geometrically symmetric, separate andself-sufficient qualifying zones of small pore size membrane, yieldingthe highest possible integrity by redundant qualifying layers, separatedby a non-restrictive inner zone which contains the reinforcement,without diminishing the performance of the qualifying layers, in asurprisingly thin overall section.

The Example 2 membrane provided a stunning improvement in flow rate overthe standard (control) membrane of about seventy eight (78%) percent,while retaining almost the same integrity attributes in IBP and FAOP.The Mean Flow Pore (MFP), a more universally recognized method for meanpore size, of which FAOP is attempting to approximate, shows theexpected difference: a larger mean flow pore is consistent with a higherflow rate, and this indicates that there is, by the flow averagingmethod, a wider distribution of pore sizes in the Example 2 membranewhen compared to the control membrane. This does not, however, diminishthe importance of the flow improvement with essentially the same InitialBubble Point, which is a rating of the single largest pore on themembrane, and a measurement which the microfiltration industry has cometo rely upon for testing the integrity of a membrane. Thus, Example 2illustrates another advantage to the membrane of the present invention,which is the ability to produce, in a single membrane, three separatezones of performance which, when oriented by decreasing pore size, canprovide a novel, surprisingly thin section combination reinforcedprefilter and final filter, having geometric symmetry, good integrity,and very high flow rates.

Example 4

The dopes used in this example were prepared as before in the firstthree examples. The dopes were produced using Nylon 66 (Monsanto/SolutiaVydyne® 66Z) polymer. Characteristics of these dopes processed asstandard dry double layer non-reinforced membrane are given in TableIII.

TABLE III Dopes for Example 4 Dope Dope % Viscosity IBP FAOP Thickness QM.F.P. I.D. Type Solids (cp) (psig) (psig) (mils) (cc/min) (micron)97A012 “A” = Smaller 14.5 3050 51.7 65.3 7.8 30.6 0.336 Pore 97A016 “B”= Larger 12.5 1500 23.5 30.3 7.5 205.0 0.789 Pore

Another geometrically symmetric and pore size symmetric reinforced threezone membrane, with an “open” (large pore size) scrim impregnation wasprepared.

A non-woven fiber spunbonded web suitable for preparation of the presentinvention (commercially available from Ahlstrom, tradename Hollytex®,Grade# 3257), having a basis weight of nominally thirty two (32)gm/sq.meter was selected for processing. The processing method wasessentially the same as disclosed in Example 1. The differences were:Zone one pressure impregnation using large pore size dope 97A016 with animpregnation weight of about six (6) gm/sq.meter of Nylon solids. Zonetwo and three were essentially simultaneously coated with substantiallyeven layers of the small pore size dope, 97A012. In this example, thetotal coating weight delivered to the two sides was about nineteen (19)gm/sq.meter of Nylon solids, this total was substantially evenly splitbetween the two sides, so that both zones received about eight (8) toabout eleven (11) gm/sq.meter of coating. Quenching, washing, drying andtesting were as in the previous examples. The test results for thismembrane are shown in Table IV. At the same time, a control membrane wasprocessed, using the small pore size dope 97A012 in Zone one, as well asin Zones two and three. The test results for the control membrane arealso shown in Table IV.

TABLE IV Membrane Test Attributes from Examples 4 Dope Type in Zone IBPFAOP Thickness Q M.F.P. Example # “3-1-2” Roll I.D. # (psig) (psig)(mils) (cc/min) (micron) 4 “A-B-A” 97A016-05 40.5 54.0 4.7 114.0 0.554(control) “A-A-A” 97A012-06 46.1 54.9 4.4  72.2 0.498

Discussion of Example 4

A three zone, reinforced membrane having an extremely thin cross-sectionwas produced, as shown above. This example demonstrates the ability ofthe reinforced zone and the two very thin qualifying zones to provide areasonably high integrity membrane. It should be noted that thethickness of the filled Hollytex scrim is approximately 3.5 mils.Therefore, the remaining 1.2 mils of the 4.7 mil membrane in Example 4is shared by zones two and three, leaving only about 0.6 mils ofeffective qualifying layer on each side of the reinforced zone. However,this thickness was sufficient to provide a flow rate improvement ofabout fifty eight (58%) percent with only about a twelve (12%) percentloss of integrity as compared to the control membrane.

Example 5

The dopes used in this example were prepared as before. The dopes wereproduced using Nylon 66 (Monsanto/Solutia Vydyne® 66Z) polymer.Characteristics of these dopes processed as standard dry double layernon-reinforced membrane are given in Table V:

TABLE V Dopes for Example 5 Dope % Viscosity, IBP FAOP Thickness QM.F.P. Dope I.D. Type Solids (cp) (psig) (psig) (mils) (cc/min) (micron)97B024 “A” = Smaller 14.5 2894 60.5 73.5 6.8  28.9 0.322 Pore 97B011 “B”= Larger 12.5 1400 29.3 36.1 6.3 136.9 0.646 Pore

Another geometrically symmetric and pore size symmetric reinforced threezone membrane, with an “open” (large pore size) scrim impregnation wasprepared as described below.

The same substrate was used as in Example 4 (Hollytex®3257) and theprocessing method was essentially the same as disclosed in Example 1.However, zone one was pressure impregnated using large pore size dope97B011 with an impregnation weight of about 6 gm/sq.meter of Nylonsolids. Zone two and three were simultaneously coated with substantiallyeven layers of the small pore size dope, 97B024. In this example, thetotal coating weight delivered to the two sides was about 38 gm/sq.meterof Nylon solids. The total coating weight delivered was split betweenthe two sides, so that both zones two and three received about seventeen(17) to about twenty one (21) gm/sq.meter of Nylon solids coating. Thegrand total application of both dopes (large and small pore size) wasthus approximately 44 gm/sq.meter Nylon solids. Quenching, washing,drying and testing were conducted as previously described. The testresults for the resulting membrane are shown in Table VI. During thesame experiment, a control membrane was processed, using the small poresize dope 97B024 in all three zones. The test results for the controlmembrane are also shown in Table VI.

TABLE VI Membrane Test Attributes from Examples 5 Dope Type in Zone IBPFAOP Thickness Q M.F.P. Example # “3-1-2” Roll I.D. # (psig) (psig)(mils) (cc/min) (micron) 5 “A-B-A” 97B024-05 61.6 75.8 6.1 45.7 0.357(control) “A-A-A” 97B024-02 64.5 79.3 6.0 29.8 0.332

Discussion of Example 5

As can be seen, as compared to example 4, the nominally higher coatingweights used to form the qualifying zones two and three in the presentexample, example 5, resulted in a very high integrity membrane having anIBP within about five (5%) percent of the control membrane, and a flowrate improvement of about fifty three (53%) percent as compared to thecontrol membrane.

Example 6

The dopes were prepared as previously described. The dopes were producedusing Nylon 66 (Monsanto/Solutia Vydyne® 66Z) polymer. Characteristicsof these dopes processed as standard dry double layer non-reinforcedmembrane are given in Table VII:

TABLE VII Dopes for Example 6 Dope Dope % Viscosity IBP FAOP Thickness QM.F.P. I.D. Type Solids (cp) (psig) (psig) (mils) (cc/min) (micron)97B066 “A” = Smaller 14.5 2980 71.8 >90 4.5 24.8 0.219 Pore 97B067 “B” =Larger 12.5 1772 31.8 39.3 5.6 93 0.628 Pore

Still another geometrically symmetric and pore size symmetric reinforcedthree zone membrane, with an “open” (large pore size) scrim impregnationwas prepared.

The same substrate as Example 4 was used, (Hollytex® 3257). Theprocessing method was essentially the same as disclosed in Example 1.However, zone one was pressure impregnated using a large pore size dope97B067 with an impregnation weight of about 6 gm/sq.meter of Nylonsolids. Zone two and three were simultaneously coated with substantiallyeven layers of the small pore size dope, 97B066. In this example, thetotal coating weight delivered to the two sides was about 24 gm/sq.meterof Nylon solids. The total coating weight delivered was split betweenthe two sides, so that both zones received about 11 to about 13gm/sq.meter of Nylon solids coating. The grand total application of bothdopes (large and small pore size) was thus approximately 30 gm/sq.meterNylon solids. Quenching, washing, drying and testing were accomplishedas before. The test results for this membrane are shown in Table VIII.During the same experiment, a control membrane was processed, using thesmall pore size dope 97B066 in all three zones. The test results for thecontrol membrane are also shown in Table VIII.

TABLE VIII Membrane Test Attributes from Examples 6 Dope Type in ZoneIBP FAOP Thickness Q M.F.P. Example # “3-1-2” Roll I.D. # (psig) (psig)(mils) (cc/min) (micron) 6 “A-B-A” 97B066-01 71.0 >90 4.6 39.6 0.261(control) “A-A-A” 97B066-11 71.7 >90 4.5 29.4 0.254

Discussion of Example 6

Again, as compared to Example 4, the nominally higher coating weights ofqualifying zones two and three in the present example have resulted in avery high integrity membrane, having an IBP within about one (1%)percent of the control membrane, and a flow rate improvement of aboutthirty five (35%) percent over the control membrane.

This particular example is representative of a 0.1 micron membrane,suitable for use in purifying water for manufacture of semiconductorsand integrated circuits, in the electronics industry. The increasedclean water flow rate of the new membrane resulting from the new processdescribed in the present application allows for the design of a smallerand less costly water treatment system in constructing a semiconductorfabrication plant, while providing the same high quality finish water atthe design demand flow rate.

Summary of Examples

The three zone membranes of the present invention are characterized ashaving markedly improved flow rates in filtration applications, fortheir pore size attributes, as compared to standard products now commonin the membrane filtration industry. The relatively thin cross-sectionsof these three zone, membrane products result in membrane cartridgeshaving more surface area and even higher throughputs. This translatesinto a higher value added product for the filtration customer.

It is believed that routine experimentation with substrates,pre-treatments, zone coating weights, polymers, dope viscosity,thickness, pore sizes, and orientations of the zones with respect topore sizes will yield optimized membrane products which have superiorperformance to existing membrane products. Other membrane applicationswhich will benefit from the ability to customize zone performance willinclude (as examples), diagnostic products using body fluids, transfermembranes, separation devices, medical devices, and others which willbecome obvious to those skilled in the arts of membrane science.

As clearly shown in FIGS. 4a-h, the three zone, supported, microporousmembrane of the present invention has three distinct, continuous, zones.Also, as clearly shown FIGS. 4b-4 d, at least one portion of the scrimencapsulated in the center zone (zone having largest pore size) at leastpartially protrudes into both the upper and the lower zones (zoneshaving the same, smaller pore size).

As clearly shown in FIGS. 5a-h, the three zone, supported, microporousmembrane made in accordance with the present invention has threedistinct, continuous, zones. Also, as clearly shown FIGS. 5b-5 d, atleast one portion of the scrim encapsulated in the center zone (zonehaving largest pore size) at least partially protrudes into the lowerzone (zone having the smaller pore size).

Based on the above, it should be clear that the teachings of the presentinvention which includes the intermingling of the dopes in fluid formfrom the three dies prior to quench provides the three zone, continuousmembrane, as described herein.

Based on the foregoing description, it should now be apparent that theuse of the apparatus and the process to produce the three zoned,reinforced membrane described herein will carry out the objects setforth hereinabove. It should also be apparent to those skilled in theart that the process of the present invention can be practiced tomanufacture a variety of microporous membranes having at least a singlelayer of support material at least substantially embedded in a firstzone of microporous membrane and having at least one zone of microporouspolymer membrane on each opposed surface of the first zone. Similarly,the dope quench solutions, concentration and temperatures thereof aswell as the speed at which the scrim is continuously fed through theapparatus can readily be determined by those skilled in the art.

It is important to note that the three zone membrane of the presentinvention has a discontinuous pore structure with a continuousentanglement of the separate layers/zones of polymer such that thecontinuous microporous membrane produced is structurally integral.

After formation of the three zone, reinforced, microporous membrane 10of the present invention, the membrane may be treated in accordance withU.S. Pat. No. 4,473,474, the disclosure of which is herein incorporatedby reference, to produce a cationically charge modified microporousmembrane particularly suitable for the filtration of parenteral orbiological liquid or, in accordance with U.S. Pat. No. 4,473,475, toproduce cationically charge modified microporous membrane particularlysuitable for the filtration of high purity water required in themanufacture of electronic components, the disclosure of each isincorporated herein by reference.

While experiments have not as yet conducted to verify that the presentinvention will have the same or similar results when using other ternaryphase inversion polymers, it is presently believed that the presentinvention can be useful in the processing of a large number of ternaryphase inversion polymers into membrane or other useful purposes becauseof the similar chemical compositions and structures. Specifically, sincenylon 66 is a member of a group of polymers that are capable of beingprocess into microporous membrane via the phase inversion process, thenature of this process is such that there is a strong probability thatthe methods and systems of the present invention will be applicable tothese other polymers as well, including, but not limited to, nylon 66,nylon 46, nylon 6, polysulfone, polyethersulfone,polyvinylidenediflouride (PVDF) and other ternary phase inversionpolymers that form microporous structures through the phase inversionprocess.

While the articles, apparatus and methods for making the articlescontained herein constitute preferred embodiments of the invention, itis to be understood that the invention is not limited to these precisearticles, apparatus and methods, and that changes may be made thereinwithout departing from the scope of the invention which is defined inthe appended claims.

What is claimed is:
 1. A three zone, reinforced, continuous,geometrically symmetrical, microporous membrane prepared by a processcomprising the acts of: providing a porous support material; providingfirst, second, and third polymeric dopes, the first dope beingformulated to produce a quenched polymeric layer having a pore sizewhich differs from quenched layer pore sizes obtained from at least oneof the second and third dopes; substantially impregnating the poroussupport material with the first polymeric dope; coating the second dopeover a first side of the impregnated porous support material such thatthe first and second dopes intermingle at an interface between the firstand second dopes; coating the third dope over a second side of theimpregnated porous support material such that the first and third dopesintermingle at an interface between the dopes; and quenching the coatedsubstantially impregnated support material thereby forming thereinforced, continuous, geometrically symmetrical three-zonedmicroporous membrane, the microporous membrane comprising: a middlemembrane zone having a first pore size, the middle zone comprising theporous support material and a microporous polymer derived from quenchingthe first polymeric dope; a second membrane zone having a second poresize, the second zone comprising a microporous polymer derived fromquenching the second polymeric dope upon the first side of thesubstantially impregnated porous support material; and a third membranezone having a third pore size, the third zone comprising a microporouspolymer derived from quenching the third polymeric dope upon the secondside of the impregnated porous support material, wherein the first poresize being at least about 20% greater than at least one of the secondand the third pore size.
 2. The microporous membrane of claim 1 wherein,during the impregnating act, all of the support material is completelycovered by the first dope.
 3. The microporous membrane of claim 1wherein, during the impregnating act, at least one side of the supportmaterial has portions thereof not completely covered by the first dope.4. The microporous membrane of claim 1 wherein, during the impregnatingact, both sides of the support material have portions thereof notcompletely covered by the first dope.
 5. The microporous membrane ofclaim 1 wherein, after the quenching act, none of the support materialprotrudes into either of the zones formed by the second dope or thethird dope.
 6. The microporous membrane of claim 1 wherein, after thequenching act, at least some portion of the support material protrudesinto at least one of the zones formed by the second dope or the thirddope.
 7. The microporous membrane of claim 1 wherein, after thequenching act, portions of the support material protrude into both ofthe zones formed by the second dope and the third dope.
 8. Themicroporous membrane of claim 1 further comprising the act of: providinga controlled vapor zone.
 9. The microporous membrane of claim 8 whereinthe controlled vapor zone prevents or at least minimizes uncontrolledatmosphere from contacting the dopes.
 10. The microporous membrane ofclaim 8 wherein the controlled vapor zone prevents dope from solidifyingon the bottom of the dies.
 11. The microporous membrane of claim 8wherein the controlled vapor zone prevents quenching of the dope beforethe dopes enter the quenching means.
 12. The microporous membrane ofclaim 1 wherein the second membrane zone and the third membrane zoneeach has an average pore size of about 10 micron or less.
 13. Themicroporous membrane of claim 1 wherein the second membrane zone and thethird membrane zone each has an average pore size ranging from about0.001 microns to about 5 micron.
 14. The microporous membrane of claim 1wherein the second membrane zone and the third membrane zone each has anaverage pore size ranging from about 0.02 microns to about 1 microns.15. The microporous membrane of claim 1 wherein the second membrane zoneand the third membrane zone have about the same average pore size. 16.The microporous membrane of claim 1 wherein the pore size of one of thesecond or the third membrane zones ranges from about 0.5 microns toabout 5 microns.
 17. The microporous membrane of claim 1 wherein thepore size of one of the second or the third membrane zones ranges fromabout 0.04 to about 10 microns.
 18. The microporous membrane of claim 1wherein the microporous polymer is formulated from a dope selected fromthe group consisting of: dopes formulated from phase inversion polymers.19. The microporous membrane of claim 1 wherein low pressure drop andhigh flow rate are exhibited across the membrane.
 20. The microporousmembrane of claim 1 wherein the membrane is monolithic.
 21. Themicroporous membrane of claim 1 wherein the average pore size of themiddle zone is at least about fifty percent (50%) greater than theaverage pore size of at least one of the second zone and the third zoneof the membrane.
 22. The microporous membrane of claim 1 wherein theaverage pore size of the middle zone is at least about 100% greater,than the average pore size of at least one of the second zone and thethird zone of the membrane.
 23. The microporous membrane of claim 1wherein the average pore size of the middle zone is at least about 200%greater than the average pore size of at least one of the second zoneand the third zone of the membrane.
 24. The microporous membrane ofclaim 1 wherein the pores of the middle zone have an average size ofabout ten (10) microns or less.
 25. The microporous membrane of claim 1wherein the pores of the middle zone have an average size of about 0.5microns to about two (2) microns.
 26. The microporous membrane of claim1 wherein the pores of the middle zone have an average size of about,0.1 to about one (1.0) microns.
 27. The microporous membrane of claim 1wherein the thickness of the middle zone is from about fifty (50)microns to about one hundred fifty (150) microns.
 28. The microporousmembrane of claim 1 wherein the thickness of the middle zone is fromabout seventy-five (75) microns to about one hundred (100) microns. 29.The microporous membrane of claim 1 wherein the thickness of the middlezone is determined by whatever dope volume is necessary to substantiallyimpregnate the scrim being impregnated at any specific time.
 30. Themicroporous membrane of claim 1 wherein the pore size of the second andthird zones range is about one (1) micron or less.
 31. The microporousmembrane of claim 1 wherein the pore size of the second and third zonesis from about 0.01 microns to about one (1) microns.
 32. The microporousmembrane of claim 1 wherein the pore size of the second and third zonesis from about 0.2 microns to about 0.5 microns.
 33. The microporousmembrane of claim 1 wherein the individual thickness of each of thesecond and third membrane zones range from about twenty-five (25)microns to about one hundred (100) microns.
 34. The microporous membraneof claim 1 wherein the individual thickness of the second and thirdzones is from about thirty-five (35) microns to about sixty (60)microns.
 35. The microporous membrane of claim 1 wherein the overallthickness of the membrane does not exceed about ten (10) mils.
 36. Themicroporous membrane of claim 1 wherein the microporous membrane isparticularly suitable for the filtration of biological or parenteralfluids.
 37. The microporous membrane of claim 1 wherein the membrane isparticularly suitable for the filtration of high purity water for theelectronics industry.
 38. The microporous membrane of claim 1 whereinthe membrane may be used with either side of the membrane facingupstream.
 39. A three zone, reinforced, continuous, geometricallysymmetrical microporous membrane prepared by a process comprising theacts of: providing a porous support material; providing fist, second,and third polymeric dopes, the first dope being formulated to produce aquenched polymeric layer having a pore size which differs from quenchedlayer pore sizes obtained from at least one of the second and thirddopes; substantially impregnating the porous support material with thefirst polymeric dope; coating the second dope over a first side of theimpregnated porous support material such that the first and second dopesintermingle at an interface between the first and second dopes; coatingthe third dope over an opposite side of the impregnated porous supportmaterial such that the first and third dopes intermingle at an interfacebetween the dopes; quenching the coated support material with aquenching means, thereby forming the reinforced, continuous,geometrically symmetrical three-zoned microporous membrane, themicroporous membrane comprising: a middle membrane zone having a firstpore size, the middle zone comprising the porous support material and amicroporous polymer derived from quenching the first polymeric dope; asecond membrane zone having a second pore size, the second zonecomprising a microporous polymer derived from quenching the secondpolymeric dope upon the first side of the impregnated porous supportmaterial; a third membrane zone having a third pore size, the third zonecomprising a microporous polymer derived from quenching the thirdpolymeric dope upon the second side of the impregnated porous supportmaterial; the first pore size being at least about 20% greater than thesecond pore size; and, the first pore size being different from thethird pore size.
 40. The microporous membrane of claim 39 wherein,during the pressure impregnating act, all of the support material iscompletely covered by the first dope.
 41. The microporous membrane ofclaim 39 wherein, during the pressure impregnating act, at least oneside of the support material has at least one portion thereof notcompletely covered by the first dope.
 42. The microporous membrane ofclaim 39 wherein, during the pressure impregnating act, both sides ofthe support material have at least one portion thereof not completelycovered by the first dope.
 43. The microporous membrane of claim 39wherein, after the quenching act, none of the support material protrudesinto either of the zones formed by the second or third dope.
 44. Themicroporous membrane of claim 39 wherein, after the quenching act, atleast one portion of the support material protrudes into at least one ofthe zones formed by the second or third dope.
 45. The microporousmembrane of claim 39 wherein, after the quenching act, portions of thesupport material protrude into both of the zones formed by the secondand third dopes.
 46. The microporous membrane of claim 39 furthercomprising the act of: providing a controlled vapor zone.
 47. Themicroporous membrane of claim 46 wherein the controlled vapor zoneprevents or at least minimizes uncontrolled atmosphere from contactingthe dopes.
 48. The microporous membrane of claim 46 wherein thecontrolled vapor zone prevents dope from solidifying on the bottom ofthe dies.
 49. The microporous membrane of claim 46 wherein thecontrolled vapor zone prevents quenching of the dope before the dopesenter the quenching means.
 50. The microporous membrane of claim 39wherein the second membrane zone and the third membrane zone each has anaverage pore size of about 10 micron or less.
 51. The microporousmembrane of claim 39 wherein the second membrane zone and the thirdmembrane zone each has an average pore size ranging from about 0.001microns to about 5 micron.
 52. The microporous membrane of claim 39wherein the second membrane zone and the third membrane zone each has anaverage pore size ranging from about 0.02 microns to about 1 microns.53. The microporous membrane of claim 39 wherein the pore size of one ofthe second or the third membrane zones ranges from about 0.5 microns toabout 5 microns.
 54. The microporous membrane of claim 39 wherein thepore size of one of the second or the third membrane zones ranges fromabout 0.04 to about 10 microns.
 55. The microporous membrane of claim 39wherein the microporous polymer is formulated from a dope selected fromthe group consisting of: dopes formulated from phase inversion polymers.56. The microporous membrane of claim 39 wherein low pressure drop andhigh flow rate are exhibited across the membrane.
 57. The microporousmembrane of claim 39 wherein the membrane is monolithic.
 58. Themicroporous membrane of claim 39 wherein the average pore size of themiddle membrane zone is at least about fifty percent (50%) greater thanthe average pore size of at least one of the second zone and the thirdzone of the membrane.
 59. The microporous membrane of claim 39 whereinthe average pore size of the middle zone is at least about 100% greater,than the average pore size of at least one of the second zone and thethird zone of the membrane.
 60. The microporous membrane of claim 39wherein the average pore size of the middle zone is at least about 200%greater than the average pore size of at least one of the second zoneand the third zone of the membrane.
 61. The microporous membrane ofclaim 39 wherein the pores of the middle zone have an average size ofabout ten (10) microns or less.
 62. The microporous membrane of claim 39wherein the pores of the middle zone have an average size of about 0.5microns to about two (2) microns.
 63. The microporous membrane of claim39 wherein the pores of the middle zone have an average size of about,0.1 to about one (1.0) microns.
 64. The microporous membrane of claim 39wherein the thickness of the middle zone is from about fifty (50)microns to about one hundred fifty (150) microns.
 65. The microporousmembrane of claim 39 wherein the thickness of the middle zone is fromabout seventy-five (75) microns to about one hundred (100) microns. 66.The microporous membrane of claim 39 wherein the thickness of the middlezone is determined by whatever dope volume is necessary to substantiallyimpregnate the scrim being impregnated at any specific time.
 67. Themicroporous membrane of claim 39 wherein the pore size of the second andthird zones range is about one (1) micron or less.
 68. The microporousmembrane of claim 39 wherein the pore size of the second and third zonesis from about 0.01 microns to about one (1) microns.
 69. The microporousmembrane of claim 39 wherein the pore size of the second and third zonesis from about 0.2 microns to about 0.5 microns.
 70. The microporousmembrane of claim 39 wherein the individual thickness of the second andthird zones is from about twenty-five (25) microns to about one hundred(100) microns.
 71. The microporous membrane of claim 39 wherein theindividual thickness of the second and third zones is from aboutthirty-five (35) microns to about sixty (60) microns.
 72. Themicroporous membrane of claim 39 wherein the overall thickness of themembrane does not exceed about ten (10) mils.
 73. The microporousmembrane of claim 39 wherein the microporous membrane is particularlysuitable for the filtration of biological or parenteral fluids.
 74. Themicroporous membrane of claim 39 wherein the membrane is particularlysuitable for the filtration of high purity water for the electronicsindustry.
 75. The microporous membrane of claim 39 wherein the membranemay be used with either side of the membrane facing upstream.